A CASE STUDY OF CONTROLLED BLASTING WITH e-DET AT BANKOLA OPENCAST MINE OF EASTERN COALFIELDS LIMITED
Bankola Open Cast Mine is working on the Jambad Top (R-VIII T) and Jambad bottom (R-VIII B) seam. The thickness of the top coal seam varies from 3.2 to 4.6m. The thickness of the bottom seam is between 6.2 to 7.0m. The overburden cover is between 17 to 24m. The inter-burden between the top and the bottom seams varies from 1.5 to 3.0 m. The OB is being removed by shovel dumper combination. 100 mm and 160 mm dia. Electric Drills are being used in 3.5 m to 6.0m high benches for blasting with Large Diameter explosives .
The Bankola UG colliery, which is around 160m from the open cast working, is developing the Bonbhal seam. The thickness of the seam is 2.7 to 3.05m. It is around 74 to 84m below surface. The mine management of Bankola Patch OCP of Eastern Coalfields Limited applied for DGMS permission for extraction of pillars of Jambad Top (R-VIII T) and Jambad Bottom (R-VIII B) seam by open cast method with deep-hole blasting and deployment of HEMM The permission was granted by Director of Mines Safety, Sitarampur granted with a condition that all the blasting operations should be carried out with the use of electronic detonators along with a scientific study of the effect of blast induced ground vibrations on the following:
a) Stability of the pillar and roof of underlying Bankola seam and Bonbhal seam of Bankola colliery.
b) Surface structures (dwelling houses and school located at 100 to 150 m) at nearby village.
Accordingly, the mine management engaged CMPDI Ltd, Asansol who in turn recommended use of electronic detonators for controlled blasting. The mine sought services of M/s Gulf Oil Corporation Limited for the supply and application of their electronic detonators (e-Det) for the above study
Electronic detonator (e-DET)
e-Det is an indigenously developed electronic detonator by M/s GOCL. e-Dets are be used in association with a programmable e-xploder device. The e-xploder transfers desired delay interval data to the firing circuit and sends coded signal to the detonators to initiate the timing sequence in the firing circuit. The e-Det is having delay number 0 & 1-30 and a maximum delay window of 3000ms. The desired delay between 1-100 ms can be set on the e-Xploder, which is custom designed to fire e-Det.
The service end of the e-Det is inserted into the cap sensitive booster charge and lowered down the hole, and the other end is connected to the surface connector. All the holes are connected in series with the surface connecter, which is finally connected to the e-Xploder for firing.
Blasting Trials and Analysis
During trial period, first round of blasts were monitored for blast vibration. The details of the results are given in Table-I. From ten blasts, fifteen readings of ground vibration levels were taken on surface at different locations near the residential structures, club and Kali temple of the village and at school buildings near ECL quarters.
The vibration monitoring stations were selected in consultation with the mine officials and the distance from the blast site to the monitoring stations were measured by the mine surveyor. The vibrations were monitored with varying explosive charge per delay and with varying distance.The minimum peak particle velocity observed on surface near the Gram Pradhan’s house at the village was 2.35 mm / sec at a distance of 170m from the blast site with maximum explosive charge per delay of 25 kg, in blasting round consisting of total explosive charge 525 kg. The frequency level was recorded as 17 Hz.
The minimum peak particle velocity was observed on pillar on Bonbahal seam as 3.30 mm / sec at a radial distance of 87.8 m from the blast site with maximum explosive charge per delay as 12.5 kg in a blasting round of 75 kg where frequency was not available.
The minimum peak particle velocity observed on roof on Bonbahal seam was 6.35 mm / sec at a distance of 114.56 m from the blast site with maximum explosive charge per delay as 19.5 kg in a blasting round of 389.2 kg where Zc frequency was 51 Hz.
The maximum peak particle velocity was observed on surface (within the mine premises) as 16.0 mm / sec at a distance of 90 m from the blast site with maximum explosive charge per delay of 25 kg in a blasting round of 750 kg where frequency level was 8Hz. The reasons for the high level of vibration were due to the excessive toe burden in the first blast. This was the first blast in the mine and prior to this the HEMM were excavating the soft strata of the mine.
The maximum peak particle velocity was observed on roof on Bonbahal seam as 25.1 mm / sec at a radial distance of 87.73 m from the blast site with maximum explosive charge per delay as 25 kg in a blasting round of 319.7 kg where Zc frequency was 85 Hz.
The maximum peak particle velocity was observed on roof on Bonbahal seam as 25.1 mm/sec at a radial distance of 87.73 m from blast site with maximum explosive charge per delay as 25 kg in a blasting round of 319.7 kg where frequency level was 85 Hz.
The following data (Table 1) was generated during the trial blast of Bankola OC patch:
BLASTWISE DETAILS OF PEAK PARTICLE VELOCITY AND AIR OVER PRESSURE IN BANKOLA MINE TRIAL BLASTS WITH e-DET
With regular blasting operations, the formation of benches in the mine became proper and the ground vibration levels in the successive blasts reduced considerably. No fly rock was observed. The fragmentation of the blasted muck was satisfactory and suitable to the excavator in use.
Further development with e-DET-
GOCL is now ready with the fully “Field Programmable’ version where individual detonators can be programmed at the field commensurate to need. The new version is having a programmable delay window of 9000 milliseconds in increments of 1 millisecond delay. This will open up tremendous opportunities to the blasting engineer while carrying out blasts, whether in mining or civil infrastructure sector.
More significantly in the Indian scenario, use of e-DETs will help unlock millions of tonnes of coal currently locked up in the opencast mine benches presently un-mineable, due to proximity to habitations. DGMS is presently not allowing mining of benches within 100 meters to dwellings. With the use of e-DET, there exists a potential to reduce this distance to 50 meters, thus helping mining of millions of ones of coal which otherwise would have been left un-mined in the pits.